Textures with alpha:

This is good for patches of texture. You need to worry about the
edges of the field. If you have a plane partially covering the screen along
the edge of the plane will look strange because it will be a hard edge. One
way to get around this is use an image-map with alpha to create an irregular
shape. You will also probably want to use multiple layers of this technique
for images at different distances.

Basic Textures with alpha:

Create a template image with something(I used the gimp)

Create a Plane in your scene
and give it a simple cloud texture using alpha

You should have something like this

Add our template as a stencil to the texture

Finally we have

Things to try:

Use an empty to animate the textures, loc,size,rotation.

Use key-frames on the materials settings.

Duplicate your object and offset or scale it slightly larger
then apply a mostly transparent material to it to get around the 8 texture
limit.